Research On Preparation And Properties Of Chromized And Chromium Oxide Layers On TC4 Alloy By Double Glow Plasma Technique

Titanium alloys are widely used in many fields, such as aerospace, navigation, and medical treatment, due to high strength-to-density ratio, good corrosion resistance, and excellent biocompatibility. However, the further applications are limited owing to their low high-temperature oxidation resistance, poor hot corrosion resistance and poor corrosion resistance for reducing solution. Therefore, it was proposed in this dissertation to prepare chromizied layer and Cr-O modified layer on TC4 alloy by using the double glow plasma alloying technique, in order to improve high-temperature resistance oxidation, hot corrosion resistance and electrochemical corrosion resistance.The chromized layer and Cr-O modified layer on TC4 alloy was fabricated by using the double glow plasma alloying technique in this paper. Orthogonal experiments of L9 (3⁴) were carried out for preparation of chromized layer, and the optimum technological parameters were obtained as follows: source voltage 950V, workpiece voltage 300V, air pressure 35Pa, distance between two cathodes 12mm, holding time 2h; While the chosen technological parameters for Cr-O modified layer were: source voltage 600V, workpiece voltage 450V, air pressure 35Pa, distance between two cathodes 12mm, ratio of oxygen and argon gas pressure 1:6, chromizing time 3h, holding time 2h. The microstructure of the prepared modified layers was characterized respectively by means of scanning electron microscope (SEM), energy diffraction spectroscopy (EDS) and X-ray diffraction (XRD). The results showed that the modified layers were uniform, compact and continuous. Besides, the chromized layer was made up of Cr, Ti and Al_1+xTi_1-x, while Cr-O modified layer was composed of Cr, Cr₂O₃ and Cr₂Ti. The thermal shock test and scratch test showed that as-prepared modified layers were both closely bonded to the substrate.It has been systematically studied for high-temperature oxidation resistance, hot corrosion resistance and electrochemical corrosion resistance of the as-fabricated chromized layer and Cr-O modified layer on TC4 alloy. High-temperature oxidation tests at 800℃showed that oxidation mass gain rate of the modified layer made at the optimum technological parameters was much lower than that of TC4 substrate. Because compact Cr₂O₃ generated at high-temperature oxidation could prevent from further incursion of oxygen effectively, greatly improving high-temperature oxidation resistance of the substrate. The hot corrosion tests showed the two modified layers were both made up of Cr₂O₃, TiO2 and (Cr_0.88Ti_0.12)₂O₃, and the mass gain of the modified samples after hot corrosion were gradually increased, exhibiting good hot corrosion resistance. The electrochemical tests showed that the corrosion resistance of chromized layer and Cr-O modified layer were greatly improved obviously.